In primary effusion lymphoma cells, MYB transcriptional repression is associated with v-FLIP expression during latent KSHV infection while both v-FLIP and v-GPCR become involved during the lytic cycle

The Role of miRNAs in Virus-Mediated Oncogenesis

To date, viruses are reported to be responsible for more than 15% of all tumors worldwide. The oncogenesis could be influenced directly by the activity of viral oncoproteins or by the chronic infection or inflammation. The group of human oncoviruses includes Epstein–Barr virus (EBV), human papillomavirus (HPV), hepatitis B virus (HBV), hepatitis C virus (HCV), human herpesvirus 8 (HHV-8) or polyomaviruses, and transregulating retroviruses such as HIV or HTLV-1. Most of these viruses express short noncoding RNAs called miRNAs to regulate their own gene expression or to influence host gene expression and thus contribute to the carcinogenic processes. In this review, we will focus on oncogenic viruses and summarize the role of both types of miRNAs, viral as well as host’s, in the oncogenesis.

KSHV miRNAs decrease expression of lytic genes in latently infected PEL and endothelial cells by targeting host transcription factors

Kaposi's sarcoma-associated herpesvirus (KSHV) microRNAs are encoded in the latency-associated region. Knockdown of KSHV miR-K12-3 and miR-K12-11 increased expression of lytic genes in BC-3 cells, and increased virus production from latently infected BCBL-1 cells. Furthermore, iSLK cells infected with miR-K12-3 and miR-K12-11 deletion mutant viruses displayed increased spontaneous reactivation and were more sensitive to inducers of reactivation than cells infected with wild type KSHV. Predicted binding sites for miR-K12-3 and miR-K12-11 were found in the 3'UTRs of the cellular transcription factors MYB, Ets-1, and C/EBPα, which activate RTA, the KSHV replication and transcription activator. Targeting of MYB by miR-K12-11 was confirmed by cloning the MYB 3'UTR downstream from the luciferase reporter. Knockdown of miR‑K12-11 resulted in increased levels of MYB transcript, and knockdown of miR-K12-3 increased both C/EBPα and Ets-1 transcripts. Thus, miR-K12-11 and miR-K12-3 contribute to maintenance of latency by decreasing RTA expression indirectly, presumably via down-regulation of MYB, C/EBPα and Ets-1, and possibly other host transcription factors.

Combination of arsenic and interferon-α inhibits expression of KSHV latent transcripts and synergistically improves survival of mice with primary effusion lymphomas

Background

Kaposi sarcoma-associated herpesvirus (KSHV) is the etiologic agent of primary effusion lymphomas (PEL). PEL cell lines infected with KSHV, but negative for Epstein-Barr virus have a tumorigenic potential in non-obese diabetic/severe combined immunodeficient mice and result in efficient engraftment and formation of malignant ascites with notable abdominal distension, consistent with the clinical manifestations of PEL in humans.

Methodology/Principal Findings

Using this preclinical mouse model, we demonstrate that the combination of arsenic trioxide and interferon-alpha (IFN) inhibits proliferation, induces apoptosis and downregulates the latent viral transcripts LANA-1, v-FLIP and v-Cyc in PEL cells derived from malignant ascites. Furthermore, this combination decreases the peritoneal volume and synergistically increases survival of PEL mice.

Conclusion/Significance

These results provide a promising rationale for the therapeutic use of arsenic/IFN in PEL patients.

Human herpesvirus 8 viral FLICE-inhibitory protein retards cell proliferation via downregulation of Id2 and Id3 expression

Death receptor-mediated apoptosis is potently inhibited by viral FLIP (FLICE/caspase 8 inhibitory protein) through reduced activation of procaspase 8. In this study, we show that the human herpesvirus 8-encoded vFLIP retards cell proliferation. Overexpression of vFLIP caused cell cycle arrest, with an apparent decrease of cells in the S phase. The Id (inhibitor of DNA binding) proteins are considered as dominant negative regulators of differentiation pathways, but positive regulators of cellular proliferation. The mechanisms by which Id proteins promote the cell cycle are diverse, but appear to involve affecting the expression of cell cycle regulators. RT-PCR results demonstrated that the expression of vFLIP decreased the expression levels of Id2 and Id3 as well as cyclin E and cyclin A compared with the vFLIP-null cells. These indicate that vFLIP affects cell proliferation by decreasing the expression levels of cell cycle regulatory proteins.

KSHV/HHV8-associated lymphomas

This review looks at the current state of knowledge on primary effusion lymphoma (PEL) and other Kaposi sarcoma herpesvirus (KSHV)/human herpesvirus 8 (HHV8)-associated lymphomas. In 1995, KSHV DNA sequences were identified within a distinct subgroup of acquired immunodeficiency syndrome-related non-Hodgkin lymphomas localized in body cavities and presenting as pleural, peritoneal and pericardial lymphomatous effusions. Subsequently, the spectrum of KSHV/HHV8-associated lymphomas has been expanded by the identification of cases of extracavitary solid lymphomas without serous effusions. Despite the diversification in the clinical presentation of KSHV/HHV8-associated lymphomas, the majority of the cases reported demonstrated similar morphology, immunophenotype and KSHV/HHV8 viral status. KSHV/HHV8 infection is also in multicentric Castleman disease-associated plasmablastic lymphoma. The exact oncogenic mechanisms of KSHV/HHV8 are not clearly defined. The prognosis for KSHV/HHV8-associated lymphomas is poor. Novel approaches for therapy, outside traditional chemotherapy with CHOP (cyclophosphamide, doxorubicin, prednisone, vincristine), have been suggested. These include the addition of antiviral therapy as well as inhibition of specific cellular targets.